Presentation on theme: "Lead dioxide and conc HCl. Lead dioxide, PbO 2, contains lead(IV). It is a strong oxidising agent. PbO 2 is very dark brown. “Red lead”, Pb 3 O 4, contains."— Presentation transcript:
Lead dioxide and conc HCl
Lead dioxide, PbO 2, contains lead(IV). It is a strong oxidising agent. PbO 2 is very dark brown. “Red lead”, Pb 3 O 4, contains some lead(IV) and some lead(II). It is also an oxidising agent.
A few drops of concentrated HCl are added. A gas is produced that immediately starts to turn damp starch-iodide paper blue-black. Can you name this gas?
Soon the top half of the starch-iodide paper has changed colour, while the bottom portion (which had been blue-black) is bleached white again.
Water is added to the mixture in the test tube, and a little of the liquid withdrawn. The colour visible here is probably due to the gas produced.
This liquid is added to colourless potassium iodide solution. A bright yellow precipitate forms. What substance reacts with I - to form a bright yellow precipitate?
When colourless potassium iodide solution is added to solid PbO 2 there is no reaction.
PbO 2 reacts with conc HCl to form chlorine gas (Cl 2 ) and Pb 2+ Lead chloride, PbCl 2, is sparingly soluble. Notice that all the dark PbO 2 has reacted. Also notice that the yellow colour from this solution has faded.
PbO 2 + 4H + + 2e - → Pb H 2 O 2Cl - → Cl 2 + 2e - PbO 2 + 2Cl - + 4H + → Pb 2+ + Cl 2 + 2H 2 O